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KS20: Practical Psychrometry
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KS20: Practical Psychrometry

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A practical introduction to psychrometry and the use of the psychrometric chart as a design tool.  KS20 is essential reading for those involved in designing and operating the systems that achieve comfortable environments for occupants, such as air conditioning. The publication covers the properties of atmospheric air; the basis for and the construction of the psychrometric chart; psychrometric processes such as heating, cooling, humidification and dehumidification and the equipment that is used.

Quote "from a practicing engineer: "Psychrometry is essential for air conditioning design, and the psychrometric chart is really useful to help you think about your design and how it will work (and check that it will work!)".

This publication is not intended to provide a comprehensive theoretical approach to Psychrometry but to give the newcomer to the topic enough information to understand how the psychrometric chart is constructed and how the information it provides can be used. It will show how psychrometry, the psychrometric chart and psychrometric processes are used in the practice of building services engineering.

The nature of atmospheric air and its properties is examined, together with the quantities by which it is measured and the processes by which it can be modified. The parameters commonly used to provide specifications for designs, and for measuring air conditions are considered. Having explained the construction of the psychrometric chart the various psychrometric processes and the equipment used to provide them are explored. These include the changes which air undergoes as it passes through an occupied space and the engineered processes such as heating, cooling,humidification, dehumidification and mixing.

With the basics and processes covered there is a brief overview explaining how the natural external environment can be transformed into a controlled internal environment by using items of equipment that can vary the temperature and humidity of air within air conditioning systems. This section is limited to describing some typical examples of sample systems. It is intended that the reader should understand the basic operation of the systems but it does not address the design process in detail.


Symbols and units

Part 1: Psychrometry and the psychrometric chart

1 Introduction

1.1 Why you need to know about the psychrometric chart

2 Psychrometric fundamentals

2.1 Humid air
2.2 Dry air and its composition
2.3 Atmospheric water vapour

3 The psychrometric chart

3.1 Dry bulb temperature
3.2 Moisture content
3.3 Percentage saturation and relative humidity
3.4 Wet bulb temperature
3.4.1 Sling wet-bulb
3.4.2 Screen wet-bulb
3.4.3 The psychrometric equation
3.5 Specific enthalpy
3.6 Specific volume and density
3.7 Dew point temperature
3.8 Plotting state points on the psychrometric chart
3.9 The CIBSE psychrometric chart
3.10 Psychrometric calculations
3.11 Using the chart for psychrometric processes
3.11.1 Sensible heating and cooling
3.11.2 Humidification and dehumidification

Part 2: Using the psychrometric chart in practice

4 Design conditions

4.1 Thermal comfort
4.2 Internal design conditions
4.3 External design conditions

5 Psychrometric processes

5.1 The room process
5.2 Mixing air streams
5.3 Air heating
5.4 Air cooling
5.5 Air humidification and humidifiers
5.5.1 Water spray types
5.5.2 Steam humidifiers
5.5.3 Psychrometric processes
5.5.4 Humidifier load
5.6 Heat recovery processes and heat exchangers
5.6.1 Effectiveness/efficiency
5.6.2 Unit types

6 Applied Psychrometry

6.1 Centralised systems
6.1.1 All-air system using all outside air
6.1.2 All-air systems with recirculation
6.2 Unitary systems
6.2.1 Dry unit operation
6.2.2 Wet unit operation
6.3 Local systems

7 Further reading


Authors: Mike Farrell & Gay Lawrence Race

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